Neuroblastoma (NB) is a common and deadly solid tumor in children that displays heterogeneous clinical behavior, from spontaneous regression to relentless progression. Evidence suggests that the TRK family of neurotrophin receptors plays a critical role in this behavior. NBs expressing TrkA are biologically favorable and prone to spontaneous regression or differentiation, whereas, TrkB-expressing tumors are extremely aggressive and often fatal tumors. We hypothesize that structural differences between TrkA and TrkB lead to differences in signaling and gene induction that result in these very different clinical behaviors. Furthermore, Trk inhibition will be an effective adjunct to treating these tumors, with a wide therapeutic index. Aim 1: We will determine the unique features of receptor structure that result in differences in protein association or signaling, which in turn cause the different biological effects of TrkA vs. TrkB activation in human NBs. We plan to examine differences between TrkA and TrkB protein associations in response to ligand binding that may lead to differences in the signaling pathways or duration of signaling; use mutation or deletion analysis to confirm the receptor domain(s) responsible for these different biological responses; and analyze gene expression patterns induced by ligand activation of TrkA or TrkB to identify critical pathways that contribute to favorable and unfavorable behaviors, respectively. Aim 2: We will compare currently available and novel Trk inhibitors to identify the optimum agent for incorporation into clinical trials. We plan to: treat TrkA and TrkB expressing NB cells in vitro and in vivo with Trk-specific tyrosine kinase inhibitors from several biotechnology companies to determine the effects on cell survival, growth, differentiation and tumorigenicity; treat Trk-expressing NBs in vitro and in vivo with novel Trk inhibitors in combination with conventional cytotoxic agents to determine optimum combinations, dose and schedule; and treat NBs in vitro and in animal models with Trk inhibitors combined with selective downstream inhibitors of PI3K/AKT, RAS, MAPK, or mTOR to determine if the combination has a greater antitumor effect than either agent alone. Aim 3: We will determine the pattern of expression of Trk family genes in primary NBs, and determine the clinical consequences of this expression. We plan to screen 805 representative primary NBs for the pattern of Trk family gene expression, and determine the predictive value of this expression pattern in selected patient subsets. We will also use this information to identify relapsed patients who would be candidates for Trk-targeted therapy. The successful completion of these studies will clarify structural differences in TrkA or TrkB that lead to differences in biological function and clinical behavior. We will determine the efficacy of Trk inhibitor therapy, alone or combined with other agents, on NBs in animal models. Finally, we will determine the pattern of Trk expression in primary NBs to predict outcome and to identify candidates for Trk inhibitor therapy. Trk receptors are expressed in other pediatric and adult tumors, so these studies will have broader implications for them as well. PUBLIC HEALTH RELEVANCE: Neuroblastoma is a common and lethal childhood cancer, and we need novel therapies that are more effective and less toxic. Our studies will provide valuable insight into the important role of the TrkA and TrkB receptors in regulating clinical behavior. We will also determine the optimum Trk inhibitor (agent, dose, schedule) to incorporate into future clinical trials, and we will determine which tumors express Trk receptors to identify who would benefit from Trk inhibitor therapy.